A treatment to counteract dysfunction of natural joints by causes such as ankylosis, osteoarthrosis, tumour or developmental disorders is to partially or completely replace the natural joint by a prosthesis.
For the treatment of dysfunctions of the temporomandibular joint, maxillofacial surgeons generally prefer a total temporomandibular joint prosthesis as identified in the preamble to preclude any risk of intrusion of the artificial condylar head into the midcranial fossa through the fossa roof or the possibility of condylar head resorption.
The natural temporomandibular joint allows both rotation and essentially horizontal translation of the mandibular condyle. Translation is controlled by lateral pterygoid muscles which are attached to the mandibular condyle. The function of these muscles is lost when the mandibular condyle to which they are attached is replaced by a mandibular prosthesis-part.
Examples of such a total temporomandibular joint prostheses are described in: `Total Prosthetic Replacement of Temporomandibular Joint ` by C. D. Kiehn et al. in Ann Plast Surg, 1979; 2:5 and in: `Temporomandibular Joint Condylar Prosthesis: A Ten Year Report `, by J. N. Kent et al. in Journal of Oral Maxillofacial Surgery,1983; 41:245-254 . In these and other known prostheses of this type, the anatomical shape of the natural articulating surfaces, i.e. the mandibular condyle, the articular eminence and the glenoid fossa, is essentially copied.
After implantation of a known prosthesis as discussed above, the mandible can only rotate about a fixed point at the side of the prosthesis, which point essentially corresponds with the centre of the replaced natural mandibular condyle, so instead of the muscular control of the translatory movement of the jaw a fixed point of rotation is obtained, precluding translatory movement of the mandible (lower jaw) at the side where the prosthesis is implanted.
This is uncomfortable for the patient and impairs the function of the mandible. Furthermore, rotation of the jaw about a substantially vertical axis is caused when a translation occurs in the opposite joint. This unnatural rotation causes exertion of abnormal loads on the opposite joint and thus an increased risk of dysfunction of the opposite natural joint. This is particularly undesirable when the prosthesis is implanted in a patient suffering from a disease which tends to affect the joints in general. Because of the above-described disadvantages of known temporomandibular joint prostheses, temporomandibular joint replacements are seldom performed in practice.